The invention relates to a method of producing acid fluorides from acid chlorides by contacting with hydrogen fluoride adducts of nitrogen-base hydrofluorides.
Acid fluorides, for example carboxylic acid fluorides and sulphuryl fluoride, are advantageous products for use per se or as intermediate products. Sulphuryl fluoride, for example, can be used as a fumigant for vermin, e.g. in wood used in buildings, churches, museums, silos and buildings, and to counteract discolouring enzymes, fungi or pathogens in wood which has not been used for building, e.g. freshly felled wood. Carboxylic acid fluorides are valuable intermediate products in chemical synthesis. Sulphuryl chloride fluoride can be reacted to form sulphuryl fluoride.
German Offenlegungsschrift DE-OS 28 23 969 discloses the preparation of fluorocarbonyl compounds from chlorocarbonyl compounds, for example of chlorodifluoroacetyl fluoride from chlorodifluoroacetyl chloride and HF adducts of hydrofluorides of organic nitrogen bases as fluorination agents. If HF adducts are used, amine is added to bind HF, or solvents having sufficient alkalinity are used to bind HF.
It was an object of the present invention to devise an improved fluorination process for the preparation of acid fluorides from acid chlorides. This object is achieved by the method of the present invention.
The method according to the invention provides for acid fluorides to be prepared from acid chlorides by contacting with hydrogen fluoride adducts of hydrofluorides of organic nitrogen bases or ammonium fluoride. In this case, the method is carried out so that, unlike in the prior art, no base which has an HF-binding effect is added. Nor is any solvent which binds HF used. At best, if in accordance with an embodiment to be described hereafter an acid is added, such as trifluoroacetic acid, this acid or part of this acid may be neutralised by base, by adding a desired quantity of base before, during or after the addition of acid. However, HF is not bound.
In addition to HF adducts of ammonium fluoride, the HF adducts of hydrofluorides of organic nitrogen bases named in German Offenlegungsschrift DE-OS 28 23 969 can be used for contacting. They can be expressed by the formula B.(HF)x, wherein B represents an organic nitrogen base and x is a whole number or a fraction from  less than 1 to 4, preferably 2-3.
Any possible primary, secondary and/or tertiary amines including N-heterocycles may be used as organic nitrogen bases B. If these amines are represented by the formula:
R1R2R3N
the meanings therein could be as follows:
The radicals R2 and R3 may be identical or different. Two of the radicals R1 and R2 or R3 may also be closed to form a cycloaliphatic ring, which may optionally be interrupted by other heteroatoms such as oxygen atoms. Likewise, it is possible for the three radicals R1, R2 and R3 to be constituents of a heterocyclic ring, which means that corresponding N-heterocycles then result. Preferred organic nitrogen bases B are primary, secondary and/or tertiary amines having a total of up to 12 C atoms, the secondary and/or tertiary aliphatic amines being particularly preferred.
Concrete examples of the bases B are:
N-butylamine, N-decylamine, diethylamine, di-n-octylamine, trimethylamine, triethylamine, tri-n-propylamine, isopropyl diethylamine, tri-n-butylamine, cyclohexylamine, N-methylaniline, N,N-dimethylaniline, pyrrolidine, piperidine, N-methylpiperidine, morpholine, pyridine, quinoline, etc.
The HF adducts of the hydrofluorides of the nitrogen bases B can easily be obtained from the bases B and hydrogen fluoride; they are low-melting substances, or substances which are liquid at room temperature, with considerable thermal loading ability. The tris-hydrofluorides can even be vacuum-distilled in non-decomposed form.
Preferably HF adducts of primary, secondary or tertiary amine hydrofluorides having up to 15 C atoms are used, in particular secondary and tertiary amine hydrofluorides. HF adducts of tri-n-propylamine hydrofluoride, tri-iso-propylamine hydrofluoride, tri-n-butylamine hydrofluoride, pyridine hydrofluoride, piperidine hydrofluoride or N,N-dimethylamine hydrofluoride are particularly well suited.
The reaction is performed in liquid phase.
The method according to the invention may be performed batchwise or continuously. In the continuous procedure, the procedure may consist of also feeding in HF, fresh HF adduct or both in addition to the acid chloride to be fluorinated. Correspondingly, reaction mixture is separated off or gaseous reaction products are distilled off or removed in gaseous form.
The inventors have discovered that HCl becomes enriched in the reaction mixture over time. If HF is not fed in intermittently or continuously, the reaction mixture or the HF adduct becomes depleted in HF. It has been discovered that the HF adduct can be regenerated by treating with HF, optionally at elevated temperature (80 to 120xc2x0 C.) and elevated pressure (e.g. autogenous pressure in an autoclave). In so doing, it was shown that it is not necessary completely to expel any HCl present. A residual content of e.g. less than 5% by weight, preferably less than 2% by weight, HCl is acceptable.
According to one embodiment of the invention, sulphuryl fluoride as acid fluoride is prepared from sulphuryl chloride or sulphur dioxide and chlorine (then under pressure for liquefaction). Here, in order to release HCl, an acid is added, for example a halogenated carboxylic acid such as trifluoroacetic acid. A nitrogen base may be added for (partial) neutralisation of this added acid. In that case, this base expediently corresponds to the base contained in the hydrofluoride.
Sulphuryl fluoride may also be prepared from sulphuryl chloride fluoride. Thus two-stage preparation of sulphuryl fluoride is possible. The first stage comprises the preparation of sulphuryl chloride fluoride from sulphuryl chloride. The ratio of amine (or NH3) to HF in the reaction mixture is not limited in this stage; it can also be performed with a very high HF content, e.g. at a ratio of amine to HF of 1:3 up to 1:10 or even above. The second stage, the fluorination of sulphuryl chloride fluoride to sulphuryl fluoride, requires a ratio of amine to HF which is greater than 1:3; for example, it is between 1:2 and 1:3. This proviso in terms of the maximum content of HF in the reaction mixture also applies to one-stage preparation of SO2F2 from SO2Cl2, if a good yield of SO2F2 is to be obtained. It is assumed that for a ratio of amine to HF of less than 1:3 the reactivity (nucleophilic character) of the Fxe2x88x92anion changes.
The two-stage preparation of sulphuryl fluoride makes it possible to use a particular method variant, because it has been established that it is simultaneously possible to regenerate the HF adduct (as mentioned above) during the fluorination of SO2Cl2 to SO2ClF. SO2Cl2, an excess of HF and HF adduct which is to be regenerated are introduced into a reactor. At elevated pressure (e.g. autogenous pressure in an autoclave), SO2ClF and regenerated HF adduct are produced simultaneously. Gaseous HCl which forms is separated off (e.g. by letting off the superatmospheric pressure and passing though inert gas such as N2). Then HF is evaporated off, in order to bring the HF content to within the limits described above (amine : HF greater than 1:3). Then the second stage can be performed for the preparation of SO2F2. If the first and second stages are performed in the same reactor, it is thus possible always to enter the second stage with re-fluorinated HF adduct.
According to another embodiment, carboxylic acid fluorides of Formula I
xe2x80x83RC(O)F
are prepared in which R stands for C1-C7-alkyl; or for C1-C7-alkyl substituted by at least 1 chlorine atom and/or by at least 1 fluorine atom. Particularly preferably, R stands for C1-C3-alkyl; or for C1-C3-alkyl substituted by at least 1 chlorine atom and/or by at least 1 fluorine atom. Very particularly, R stands for CH3, C2H5, CF3, CF2Cl, CFCl2, CCl3, CHF2, C2F5 or C3F7.
R may however also stand for aromatic radicals such as phenyl or tolyl.
The chlorine-fluorine exchange according to the present invention is preferably performed at a temperature from ambient temperature (about 20xc2x0 C.) to 150xc2x0 C. Preferably the molar ratio of the HF adduct of the hydrofluoride, relative to the base contained therein, to acid chloride lies in the range from 1:0.01 to 1:1 (1 mole R3N.2.6 HF and 1 mole acid chloride are then in a ratio of 1:1), if one Cl atom is to be exchanged per acid chloride adduct. In the case of a plurality of chlorine atoms which are to be exchanged per molecule, the use of the hydrofluoride is expediently twice, three times, etc. as high. For a continuous procedure, the ratio may lie in the range from 1:0.01 to 1:100.
Upon the reaction, hydrogen chloride is released spontaneously in the preparation of carboxylic acid fluorides. This hydrogen chloride can be let off from the reactor (for example by a correspondingly adjusted pressure-relief valve). When preparing SO2F2, the addition of acids such as trifluoroacetic acid is necessary in order to release HCl.
It was established in tests that often the addition of e.g. trifluoroacetic acid right at the beginning of the fluorination reaction is advantageous insofar as the precipitation of solids (which re-dissolve again later) is prevented or reduced. For example, as little as 10 mole % of the acid, relative to the onium-HF adduct calculated as 100 mole %, is sufficient.
It was also established that ammonium salts having three C1 or C2 alkyl radicals very easily release resulting HCl; however, they tend to form solids, so that the addition of e.g. trifluoroacetic acid, as described above, is advantageous. Although onium salts having three C3- or higher-chain alkyl radicals do not form solids, HCl is not so easily released therefrom as from the shorter-chain substituted onium salts. In this case, addition of acid is advantageous, because this expels HCl more intensively.
According to one embodiment, the hydrofluoride adduct functions as a fluorination agent. It is then used in such a quantity that it is not dehydrofluorinated to such an extent that the stage of the onium monohydrofluoride is exceeded. If, for example, an adduct of the formula R3N.2.6 HF is used, only such a quantity of HF should be used that R3N.zHF with z=1 or z greater than 1 remains in the reaction mixture. The corresponding hydrochloride should not be produced if it is desired to avoid regeneration under pressure with HF for as long a time as possible. It is sufficient here merely to add HF.
If, for example, a chlorine atom is to be exchanged for a fluorine atom in a compound, at least 1 mole of the adduct R3N.2.6 HF is used per 1.6 moles of the starting compound. If other educts (e.g. SO2Cl2) or onium salts having a different HF content are used, the stoichiometry should be adapted accordingly.
According to another embodiment, the hydrofluoride adduct functions as a catalyst. Then HF is also introduced into the reaction as fluorination agent. The quantity of HF is then advantageously at least 1 mole HF per chlorine atom to be exchanged. The ratio of the total of free HF and HF bound in the adduct to the chlorine atom to be exchanged may for example lie in the range from about 1:1 up to 1:3, if SO2F2 is to be prepared from SO2Cl2 or SO2FCl. In the preparation of carboxylic acid fluorides or SO2FCl, it may be still higher if HF is to act as a solvent. It is also possible to use less than 1 mole HF per chlorine atom to be exchanged; then the HF from the HF adduct which per se has a catalytic effect is consumed, and the yield drops.
Compared with the known method, the hydrofluoride of ammonia or of the organic nitrogen base does not function here as a fluorination agent and reaction partner, but as a catalyst. For this reason, a continuous procedure becomes possible for the first time. The hydrogen chloride released may if desired be let off continuously from the reaction system or be removed upon regeneration. It is not necessary to use a solvent.
Thus the method according to the invention has the advantage that working-up is very much easier: no amine hydrochloride is produced as waste product; and a solvent does not need to be separated off.
A further subject of the invention is a composition which has a fluorinating action. It is obtainable by mixing HF adducts of the formula B1.mHF and an acid. This has the effect of releasing HCl from the reaction mixture during the preparation of SO2F2. Optionally, base B1 may also be added, in a quantity less than the quantity necessary to neutralise the acid. B1 stands for NH3 or the base B, as defined above, and m is 1 less than m less than 4. The preferred acid is a halogenated carboxylic acid, such as trifluoroacetic acid. The HF adduct of B1 may also be produced in situ. If desired, this acid or part of the acid may be introduced in the form of the salt with B1. The preferred composition has the xe2x80x9cformulaxe2x80x9d B1.(0.1-1.0)TFA.(1.0-3.0)HF. TFA is trifluoroacetic acid. B1 is preferably B. The preferred B is set forth above.
The use of HF adducts of onium salts of nitrogen as fluorination catalyst for the fluorine-chlorine exchange and the fluorine-bromine exchange in carbon atoms activated by additional electronegative substituents, in particular in C(O)Cl and C(O)Br groups, is likewise a subject of the invention.